It is believed to be of great importance in the design of prosthetic limbs and their controllers that the limbs look and operate as much like actual human limbs as possible. Generally, amputees are disinclined to use prosthetic devices that appear unnatural in look or operation or which require unnatural movements to operate. Prosthetic limbs, such as the Liberty Mutual Boston elbow system, are able to provide limited functions for amputees corresponding to the functions of the original limb.
The operation of some prosthetic limbs is controlled, at least partially, by myoelectric signals of remnant muscles, and preferably a pair of agonist-antagonist muscles, remaining after amputation. The myoelectric signals are picked up by surface electrodes placed on the skin of the amputee adjacent to the muscle remnants. Responsive to the myoelectric signals, the prosthetic limb controller controls the flow of energy from a battery to one or more motors actuating the prosthesis to perform a specified function.
The most advanced prosthetic limbs are able to provide a plurality of degrees of freedom. For instance a prosthetic arm may provide the following three degrees of freedom; (1) wrist pronation and supination, (2) elbow flexion and extension, and (3) hand closing and opening. Each degree of freedom comprises two functions. For instance, the elbow degree of freedom comprising the two functions of (1) flexion and (2) extension. Ideally, a myoelectric prosthetic controller would use the myoelectric signals of the same remnant agonist-antagonist muscle pairs that control normal arms. However, this is only possible for certain below-elbow amputees.
To provide multiple degrees of freedom in a prosthetic limb for above-elbow amputees, or those with shoulder disarticulation, a prosthetic controller must have a means for switching between the various degrees of freedom in addition to means for performing the movements of the specified degree chosen. For instance, in an arm prosthesis for which the user has only biceps and triceps muscle remnants (collectively, a single pair of agonist-antagonist muscles), in one switch state, myoelectric signals of the muscle pair control elbow movements while in another switch state, the signals control hand or wrist movements. Such switching means typically comprises a mechanical switch operated by a predetermined shoulder movement. This switching scheme, however, requires unnatural participation of other body parts.
PCT application No. 008599960 discloses a method and apparatus for multifunction control of a myoelectrically controlled prosthetic limb by recognition of patterns associated with the onset of muscle contraction in a single pair of agonist-antagonist muscles. In the scheme disclosed therein, specific characteristics, such as mean absolute value, slope, zero crossings, wave length, and slope change, of the myoelectric signals associated with the initiation of a plurality of muscle contractions are generated and processed to produce representative primitives or descriptors. The classification of the descriptors can then be accomplished using any number of distance measures as is known in the syntactic pattern recognition art. Alternately, the known descriptors can be used to train a multi-layer neural network using a back propagation algorithom.
The myoelectric patterns and characteristics associated with a single pair of agonist-antagonist muscles are different for each different function. For instance, the biceps-triceps muscle pair would have a particular myoelectric characteristic associated with elbow flexion, but would have a different myoelectric characteristic signal for elbow extension.
The data in the neural network are generated during a training session in which the user performs a plurality of muscle contractions associated with a plurality of functions. Considering an above-elbow amputee having a prosthetic arm, for instance, a single-contact electrode is placed on the biceps remnant and another single-contact, large-contact-surface electrode is placed on the triceps remnant. The differential signal between the electrodes is recorded and the selected characteristics are calculated and averaged.
During normal operation subsequent to training, the controller constantly records and observes a specified interval of the myoelectric signals of the user's muscle pair and determines total energy. If the total energy exceeds a predetermined threshold in a specified interval, pattern recognition software is triggered. The signal in the triggering interval plus the myoelectric signal to follow for another specified interval are placed in a memory and the various characteristics of the stored total interval, such as mean absolute value, etc., are determined and input to the neural network. The prosthesis controller switches to operate the function corresponding to the most closely resembled pattern class and the myoelectric signals from the muscle pair are then used to control that function. The control of the function, once selected, is accomplished by means separate from the selection software and which is not of concern to the present invention.
U.S. Pat. No. 4,030,141 issued to Daniel Graupe discloses a similar scheme for switching among degrees of freedom of a multifunction prosthesis.
These prosthesis control systems are complicated and require significant data processing capacity to perform the complex mathematics needed. The microprocessor and associated circuitry is significant and may not easily fit within the prosthetic device. Further, the electrode arrangement disclosed in PCT application No. 008599960 is prone to receiving faulty signals due to capacitance changes between the two single contact electrodes. In particular, since the electrodes are placed on the opposite side of the limb from each other, muscle contractions commonly will cause the distance between the electrodes, and thus the capacitance, to change leading to interference with the pure myoelectric signals.
Accordingly, it is an object of the present invention to provide an improved prosthetic device.
It is a further object of the present invention to provide an improved prosthesis controller with multiple degrees of freedom.
It is yet a further object of the present invention to provide an improved degree of freedom selection apparatus and method for a prosthesis controller.
It is one more object of the present invention to provide an improved degree of freedom selection apparatus and method for a prosthesis controller which uses commercially available electrodes of proven reliability and signal processing circuitry of proven reliability.
It is yet one more object of the present invention to provide a switching apparatus and method for a myoelectric prosthesis controller having multiple degrees of freedom which recognizes myoelectric signals requiring greatly reduced computational overhead than is known in the prior art.
It is another object of the present invention to provide a myoelectric prosthesis controller of small size and low weight.
It is yet another object of the present invention to provide a switching apparatus and method for a prosthesis controller having multiple degrees of freedom which is highly accurate in recognizing predetermined myoelectric patterns.